Softness is an important attribute of tissue paper products. Consumers perceive soft tissue products as tactilely pleasant against the skin, and therefore desirable. Manufacturers of tissue products therefore seek to improve the perceived softness of tissue products to increase sales.
Tissue products are typically formed, at least in part, from cellulosic pulps containing wood fibers. Those skilled in the art recognize that the perceived softness of a tissue product formed from such pulps is related to the coarseness of pulp fibers. Pulps having fibers with low coarseness are desirable because tissue paper made from fibers having a low coarseness can be made softer than similar tissue paper made from fibers having a high coarseness.
Fiber coarseness generally increases as fiber length and fiber surface area increase. The softness of tissue products can be improved by forming the tissue products from pulps comprising only short fibers. Unfortunately, tissue paper strength generally decreases as the average fiber length is reduced. Therefore, simply reducing the pulp average fiber length can result in an undesirable trade-off between product softness and product strength.
Another method for reducing the coarseness of fibers comprises lengthwise slicing individual fibers with a sliding microtome. Slicing fibers lengthwise reduces the fiber weight per unit fiber length and alters the naturally occurring closed fiber wall cross-section to an open fiber wall cross-section. Such a method is disclosed in U.S. Pat. No. 4,874,465 issued Oct. 17, 1989 to Cochrane et al. Slicing fibers lengthwise requires meticulous processing and is not considered to be a commercially feasible method of providing the quantities of fibers needed for making tissue products.
Tissue products having improved softness can also be formed from pulps comprising fibers from selected species of hardwood trees. Hardwood fibers are generally less coarse than softwood fibers. For example, those skilled in the art recognize that bleached kraft pulps made from eucalyptus contain fibers of relatively low coarseness and can be used to improve the perceived softness of tissue products.
Unfortunately, virgin kraft pulps made from a single species such as eucalyptus are in relatively limited supply and are therefore more expensive than certain pulps which tend to comprise fibers generally having inferior coarseness properties. Examples include pulps which are derived by mechanical pulping regardless of the source species and recycled pulps which invariably contain a mixture of fiber types and species. The concern over the depletion of the world's forest reserve has increased interest in utilizing such recycled pulps. Recycled pulps typically contain a blend of hardwood and softwood fibers from a variety of species. Such blends are particularly prone to having relatively high coarseness compared to their average fiber length.
In addition to inferior coarseness, the above-mentioned fiber blends often suffer from an undesirable non-uniformity in fiber properties. For example, it is believed that one of the advantages of the bleached kraft pulp made from eucalyptus is that it tends to be highly uniform in coarseness in addition to having a desirable average coarseness. One index of the distribution of coarseness within a specimen of pulp fibers can be obtained by measuring and ranking the specimen fibers by fiber surface area to obtain a group of fibers within the pulp specimen comprising the largest one percent of fibers in the specimen. The surface area of the smallest surface area fiber in this group, referred to as the minimum fiber surface area, provides an index of the coarseness distribution in the pulp specimen. A comparatively low value of this minimum fiber surface area indicates that the pulp specimen is relatively uniform with respect to coarseness. A comparatively high value of the minimum fiber surface area indicates that the pulp specimen is relatively non-uniform and will be less desirable for the application at hand even if the average coarseness of the specimen is in a desirable range.
In addition, it is necessary to consider the relative content of hardwood and softwood in judging whether a particular pulp specimen has a comparatively low or high value of minimum fiber surface area. A technique for determining whether a particular sample has a comparatively high or low value of minimum fiber surface area is discussed in the specification. The measured minimum fiber surface area can be reduced by a scale factor for each percentage of softwood in the pulp specimen. This reduced minimum fiber surface area is referred to as the pulp incremental surface area. A pulp specimen having a value of incremental surface area below a threshold level is considered to be uniform with respect to coarseness.
The papermaker who is able to obtain pulps having a desirable combination of fiber length and coarseness from fiber blends generally regarded as inferior with respect to average coarseness and uniformity of fiber properties may reap significant cost savings and/or product improvements. For example, the papermaker may wish to make a tissue paper of superior strength without incurring the usual degradation in softness which accompanies higher strength. Alternatively, the papermaker may wish a higher degree of paper surface bonding to reduce the release of free fibers without suffering the usual decrease in softness which accompanies greater bonding of surface fibers.
Accordingly, one object of the present invention is to provide a cellulose pulp having a fiber coarseness less than a threshold coarseness level.
Another object of the present invention is to provide a cellulose pulp comprising a blend of softwood and hardwood fibers and having a desirable combination of fiber length and fiber coarseness.
Still another object of the present invention is to provide a method for producing a cellulose pulp having a desirable combination of fiber length and fiber coarseness.
These and other objects are obtained using the present invention, as will be seen from the following disclosure.
All percentages, ratios, and proportions herein are by weight, unless otherwise specified. All fiber weight percentages are dry weight percentages unless otherwise specified.